262
One of a few attractive options is the possible reuse of onsite rural wastewater or
treated municipal wastewater from treatment plants for agricultural and industrial
activities [32, 168]. Since these wastewaters constitute one of the largest possible
water resources, its reuse is anticipated to offset more clean water resource.
Recycling wastewaters are usually associated with the presence of suspended solids, health-threat coliforms, and soluble refractory organic compounds that are both
tedious and expensive to treat [314]. Currently available water treatment technologies such as adsorption or coagulation merely concentrate the pollutants present by
transferring them to other phases, but they will still remain and will not be completely
“eliminated” or “destroyed” [252]. Other conventional water treatment methods
such as sedimentation, filtration, and chemical and membrane technologies involve
high operating costs and could generate toxic secondary pollutants into the ecosystem [103]. These concentrated toxic contaminants are highly redundant and have
been concerned worldwide due to the increasing environmental awareness and legislation. Chlorination has been the most commonly and widely used disinfection
process. The disinfection by-products generated from chlorination are mutagenic
and carcinogenic to human health [65, 199, 339]. These have led to the rapid R&D
in the field of “advanced oxidation processes (AOPs)” as the innovative water treatment technologies. The rationales of these AOPs are based on the in situ generation
of highly reactive transitory species (i.e., H 2 O 2 , OH
•
, O 2
•−
, O 3 ) for mineralization of
refractory organic compounds, water pathogens, and disinfection by-products [86,
259]. Among these AOPs, heterogeneous photocatalysis employing semiconductor
catalysts (TiO 2 , ZnO, Fe 2 O 3 , CdS, GaP, and ZnS) has demonstrated its efficiency in
degrading a wide range of ambiguous refractory organics into readily biodegradable
compounds, and eventually mineralizing them to innocuous carbon dioxide and
water. Among the semiconductor catalysts, titanium dioxide (TiO 2 ) has received the
greatest interest in R&D of photocatalysis technology. The TiO 2 is the most active
photocatalyst under the photon energy of 300 nm < l < 390 nm and remains stable
after the repeated catalytic cycles, whereas Cds or GaP is degraded along to produce
toxic products [205]. Other than these, the multifaceted functional properties of
TiO 2 catalyst, such as their chemical and thermal stability or resistance to chemical
breakdown and their strong mechanical properties, have promoted its wide application in photocatalytic water treatment. A number of important features for the heterogeneous photocatalysis have extended their feasible applications in water
treatment, such as (1) ambient operating temperature and pressure, (2) complete
mineralization of parents and their intermediate compounds without secondary pollution, and (3) low operating costs. The fact that the highly reactive oxygen species
(ROS) are generated as a result of the photoinduced charge separation on TiO 2 surfaces for microbial inactivation and organic mineralization without creating any
secondary pollution is well documented. So far, the application of such TiO 2 catalysts for water treatment is still experiencing a series of technical challenges. The
post-separation of the semiconductor TiO 2 catalyst after water treatment remains as
the major obstacle toward the practicality as an industrial process. The fine particle
size of the TiO 2 , together with their large surface area-to-volume ratio and surface
energy, creates a strong tendency for catalyst agglomeration during the operation.
13 Wastewater
One of a few attractive options is the possible reuse of onsite rural wastewater or
treated municipal wastewater from treatment plants for agricultural and industrial
activities [32, 168]. Since these wastewaters constitute one of the largest possible
water resources, its reuse is anticipated to offset more clean water resource.
Recycling wastewaters are usually associated with the presence of suspended solids, health-threat coliforms, and soluble refractory organic compounds that are both
tedious and expensive to treat [314]. Currently available water treatment technologies such as adsorption or coagulation merely concentrate the pollutants present by
transferring them to other phases, but they will still remain and will not be completely
“eliminated” or “destroyed” [252]. Other conventional water treatment methods
such as sedimentation, filtration, and chemical and membrane technologies involve
high operating costs and could generate toxic secondary pollutants into the ecosystem [103]. These concentrated toxic contaminants are highly redundant and have
been concerned worldwide due to the increasing environmental awareness and legislation. Chlorination has been the most commonly and widely used disinfection
process. The disinfection by-products generated from chlorination are mutagenic
and carcinogenic to human health [65, 199, 339]. These have led to the rapid R&D
in the field of “advanced oxidation processes (AOPs)” as the innovative water treatment technologies. The rationales of these AOPs are based on the in situ generation
of highly reactive transitory species (i.e., H 2 O 2 , OH
•
, O 2
•−
, O 3 ) for mineralization of
refractory organic compounds, water pathogens, and disinfection by-products [86,
259]. Among these AOPs, heterogeneous photocatalysis employing semiconductor
catalysts (TiO 2 , ZnO, Fe 2 O 3 , CdS, GaP, and ZnS) has demonstrated its efficiency in
degrading a wide range of ambiguous refractory organics into readily biodegradable
compounds, and eventually mineralizing them to innocuous carbon dioxide and
water. Among the semiconductor catalysts, titanium dioxide (TiO 2 ) has received the
greatest interest in R&D of photocatalysis technology. The TiO 2 is the most active
photocatalyst under the photon energy of 300 nm < l < 390 nm and remains stable
after the repeated catalytic cycles, whereas Cds or GaP is degraded along to produce
toxic products [205]. Other than these, the multifaceted functional properties of
TiO 2 catalyst, such as their chemical and thermal stability or resistance to chemical
breakdown and their strong mechanical properties, have promoted its wide application in photocatalytic water treatment. A number of important features for the heterogeneous photocatalysis have extended their feasible applications in water
treatment, such as (1) ambient operating temperature and pressure, (2) complete
mineralization of parents and their intermediate compounds without secondary pollution, and (3) low operating costs. The fact that the highly reactive oxygen species
(ROS) are generated as a result of the photoinduced charge separation on TiO 2 surfaces for microbial inactivation and organic mineralization without creating any
secondary pollution is well documented. So far, the application of such TiO 2 catalysts for water treatment is still experiencing a series of technical challenges. The
post-separation of the semiconductor TiO 2 catalyst after water treatment remains as
the major obstacle toward the practicality as an industrial process. The fine particle
size of the TiO 2 , together with their large surface area-to-volume ratio and surface
energy, creates a strong tendency for catalyst agglomeration during the operation.
13 Wastewater